The invention relates to a method and machine to cut circuit boards or integrated circuit packages.
In recent years, in the microelectronics industry there has been a drive for lightweight low profile consumer electronics products such as laptop computers and mobile telephones. These consumer electronics products require high-throughput assembly of low-cost, low-profile and lightweight integrated circuit packages. Integrated circuits are packaged in multiple units to achieve the required throughput and to reduce handling requirements. At the end of this process it is then necessary to singulate the packaged devices.
By reducing the area of the integrated circuit package it is also possible to reduce the area of the circuit boards on which these devices will perform their function. To this end, circuit boards are becoming lighter and smaller.
The materials from which integrated circuit packages or circuit boards are fabricated may include, for example, copper layers, gills fiber layers or weave, FR4, BT glass/epoxy, adhesives, encapsulants, solder masks or semiconductor. Another type of circuit board, is a polymer-based flexible (flex) circuit. Also, the invention may be applied to cut thin layers such as liquid crystal sheets or electrochromic dielectric thin films as used in displays.
FIGS. A(a) to A(d) show examples of strips on which several BGA devices are mounted. In FIG. A(a) encapsulant material 1 protects the die 2 and the electrical connections between the die and the substrate 3. The assembly process may require the presence of tooling holes and cut out sections 4 for ease of punching. The strip of FIG. A(a) is similar to that of FIG. A(a) except that cut out strips are missing, and in the strip of FIG. A(c) there are no tooling holes. In the strip of FIG A(d) the encapsulant covers multiple dies. In this situation, BGA singulation requires that the encapsulant be cut also. The dashed lines 5 in the drawings indicate the cutting lines to singulate individual packages.
A further example of a multiple unit, chip-scale package assembly is shown in FIG. B. In this example dies are mounted on a substrate in two dimensions to form an Nxc3x97N assembly of BGA packages 6. FIG. B shows the underside of the assembly. Solder balls 7 are positioned at the correct position on the circuit board and are then reflowed. The reflowed solder balls then form the electrical contact between the circuit board and the package. The electrical connection to the die is through the package substrate. The substrate layers may be copper layers, glass fiber layers or weave, FR4, BT glass/epoxy, adhesives, encapsulants, solder masks or semiconductor. Referring to the end view in FIG. B the substrate often comprises multiple layers which may include solder mask 8, copper 9, dielectric 10, glass 11, and epoxy 12. Gold or another conductor may be used in the layer 9 instead of copper.
FIG. C shows an ample of a circuit board panel containing multiple circuit boards 21. Such panels may be those used for xe2x80x9csmart cardsxe2x80x9d or mobile telephone circuits. The circuit board material may be rigid or flexible material made from laminated layers of copper layers, glass fiber layers or weave, FR4, BT glass/epoxy, adhesives, encapsulants, solder masks, or other materials used in circuit board manufacture.
The circuit board may be of a flexible material. This type of circuit generally is made from layers of copper, adhesive and polymer such ad Kapton polyimide or another polymer with the required mechanical properties.
The electronics industry also uses liquid crystal, electro-chromic or more generally, thin film sheeting in liquid crystal displays or in mass-produced display assemblies.
Regarding the preset methods of singulation, FIG. D shows the final steps involved in the present method of manufacturing of BGA/CSP devices. Due to the nature of the devices and systems involved, several handing and cleaning steps must be added in order to support the singulation process with wafer saws. The steps include:
electrical test and laser mark in boats, panels or trays,
removal from boat or tray and mounting on tacky UV tape,
cutting with saw and cleaning,
UV cure,
placing on trays,
marketing and inspection.
Sawing and punching of chip scale packages is described in WO9903128: (Singulation system for chip scale packages) and in WO98/52212: (Pick and place cutting head that separates chip scale packages from a Multi layered film strip). Several consumables, such as UV tape, wafer rings, saw blades, and cleaning solutions must be used.
An object of the invention is to provide a system and a method to cut through the above materials at a rate sufficient to meet the singulation rate requirements for a production line.
Another object is that the method and apparatus provide a higher yield by reducing the extent of deposited debris and by reducing handling requirements.
According to the invention, there is provided a method for singulating an electronic circuit by cutting laminated material joining the circuits, the method comprising the step of:
generating a laser beam having the following properties:
a wavelength of less than 400 nm, and
a minimum energy density of 100 J/cm2 or a minimum peak power density of 1GW/cm2;
aligning the beam relative to a feature or fiducial of the material; and
training the beam along the material until a cut has been made.
In one embodiment, beam is moved to have a spatial overlap of consecutive pulses, the overlap being in the range of 5% to 95%.
In one embodiment, the overlap is in the range of 30% to 50%.
In another embodiment, the beam is moved in a plurality of passes.
In a further embodiment, the beam is moved in greater than five passes.
In one embodiment, the beam is generated with a pulse repetition rate of greater than 1 kHz.
In one embodiment, the thickness of the laminated structures may be up to the thickness defined by the depth of focus of the laser beam.
In one embodiment, the laminate material contains two or more layers selected from BT epoxy, glass fibers, copper, gold, poly-imide, adhesive, overmold materials, underfills, conductors, dielectrics, stiffeners, stabilisers, protectors or other materials as used in electronic packaging.
In another embodiment, the individual layers of the laminate material have different ablation and ionization thresholds, different abalation and ionization rates, and different non-linear absorption and non-ionization coefficients.
In a further embodiment, the beam is generated from a solid state laser with a characteristic average power peak at a specific repetition frequency.
In one embodiment, the beam is controlled so that the average power drops as the repetition frequency is increased or decreased, and although individual pulse energy may be increased at a repetition frequency other than the repetition frequency for maximum average power the maximum cut rate is achieved at a repetition frequency other than either of these frequencies due to the contribution of other laser cutting parameters.
In one embodiment, the average power of said laser beam is greater than 3W, with a pulse width less than 100 nanoseconds, a consecutive pulse spatial overlap of 10-70%, and a beam diameter less than 70 microns at the 1/e2 point of a spatial intensity profile.
In one embodiment, the laser beam is generated by a diode laser pumped gain medium device with a fundamental emission in the 900 to 1600 nm wavelength range and with second, third, fourth or fifth harmonic generation of xc2xd, ⅓, xc2xc, ⅕th of this wavelength which is obtained by placing appropriate crystals in the laser cavity or outside the laser cavity.
In one embodiment, said laser device may be of the Nd: YAG, ND: YLF, Nd: YVO4 or the other combinations of Impurity:Host gain media lasting in the required range and with harmonic generation to an operating wavelength of less than 400 nm.
In one embodiment, the beam is delivered to the work surface using one or more mirrors mounted on one or more scanning galvanometers, and in which the required spot size is achieved by use of an on-axis lens position adjustment at a stage before the galvanometer mirror, and at a stage after the galvanometer mirror by a lens of a flat field lens, or by the use of a combination of these lenses.
In one embodiment, the laser beam is delivered using one or more mirrors mounted on one or more translational stages, and focusing is achieved by the use of a telescope or an on-axis lens before the moving mirrors or lens mounted before the sample surface and moving with the beam delivery mirror such that the focussed beam is delivered to the sample surface.
In another embodiment, the beam is telescoped and focussed to achieve the required spot size at the cutting plane with the telescope or scan lens chosen such that the beam waist remains within a specified percentage of the optimum spot size throughout the range over which the beam is delivered, and where the range is greater than the thickness of the part.
In a further embodiment, an assist gas is used to assist the cutting process to prevent debris from being deposited on the material surface, and wherein the assist gas removes material generated during the cut process so that it does not create absorption or scattering of consecutive laser light pulses.
In a still further embodiment, the assist gas is used to provide an inert atmosphere to prevent unwanted specific photochemical or photo-physical reactions form occurring during cutting.
In one embodiment, a vacuum suction process is used to extract fumes and solid debris generated at the cut surface.
In one embodiment, alignment of the laser beam to a feature on the material surface is achieved by use of a sensor and means for image processing to provide the coordinates along which cutting occurs, and wherein a beam positioning mechanism is controlled to ensure that the laser beam follows the required cutting path.
According to another aspect the invention provides a circuit singulation system comprising:
means for supporting a set of electronic circuits interconnected by material;
a laser beam source comprising means for generating a laser beam having:
a wavelength of less than 400 nm, and
a minimum energy density of 100J/cm2 or a peak power density of 1GW/cm2,
a beam positioning mechanism comprising means for directing the beam a the material and for training it along cut lines to singulate electronic circuits.
In one embodiment, the beam positioning system comprises a series of mirrors, at least some of which are movable for directing the laser beam, and a focusing lens.
In one embodiment, the mirrors are linearly movable.
In another embodiment, the mirrors are rotatable.